A radiation window is a part of a measurement apparatus that allows a desired part of electromagnetic radiation to pass through. In many cases the radiation window must nevertheless be gastight, in order to seal and protect an enclosure where reduced pressure and/or a particular gas contents prevail. The pressure inside the enclosure may be also higher than outside, for example in measurement apparata on board space-going platforms. In order to cause as little absorption as possible of the desired radiation, a major part of the radiation window should consist of a very thin foil.
Beryllium is known as a very good material for radiation window foils especially in X-ray measurement apparata, because it has a low atomic number (4) and consequently exhibits very low absorption of X-rays. Another characteristic of beryllium that makes it very useful for radiation window foils is its exceptional flexural rigidity. The thinnest beryllium foils that are commercially available for use in radiation windows at the time of writing this description have a thickness in the order of 8 micrometres. According to prior art, the beryllium foil is manufactured from an ingot by rolling. Various coatings can be applied to the beryllium foil for example to enhance its gastightness and corrosion resistance as well as to keep undesired parts of the electromagnetic spectrum (such as visible light) from passing through the foil. An example of known radiation window foils is the DuraBeryllium foil available from Moxtek Inc., Orem, Utah, USA. It comprises an 8 micrometres thick beryllium foil coated with a DuraCoat coating. DuraBeryllium, DuraCoat, and Moxtek are registered trademarks of Moxtek Incorporated.
At the time of writing this description it appears that the rolling technology has met its limits in the sense that it has not been shown capable of manufacturing beryllium foils thinner than 8 micrometres so that they would still be sufficiently gastight. This phenomenon is associated with the relatively large grain size (larger than foil thickness), which results from the grain structure of the original beryllium ingot. Grain boundaries in the beryllium foil tend to cause gas leaks through the foil.
A feature of beryllium as a material that presents technical challenges is its toxicity, especially by inhalation of beryllium-containing dust. It would be advantageous if the manufacturing method and the structure of a thin radiation window would allow using also other materials in place of beryllium.
From prior art it is also known to utilise a support grid as a mechanical reinforcement of a radiation window foil. Prior art documents that discuss various aspects of support grids are at least US 2008/0296518 A1 and US 2009/0173897 A1.